1. Field of the Invention
The present invention relates to a method of producing a crosslinked polymer particle.
2. Description of the Related Art
Polymer particles having a particle diameter ranging from 0.01 to 50 μm are useful as spacers, standard particles, diagnostic particles for inspection of antigen-antibody reaction, catalyst particles, combinatorial chemistry particles, toner additives, cosmetic additives, rheology control agents, low profile additives, improvers of mechanical characteristics of resins, antiblocking agents for resin films, lubricants for films, and running stabilizers for thermosensitive paper and are used in various fields.
In recent years, the performance demanded of polymer particles has ranged over various characteristics such as not only shape and uniformity of particle diameters, but also strength and heat resistance.
For instance, a spacer of a liquid crystal display is used to keep a constant clearance between two glass substrates constituting the display, and polymer particles are frequently used as the spacer (spacer particles). However, if a distribution of particle diameters of these polymer particles is wide, spacer particles having diameters smaller than a center particle diameter move within the space between the two glass substrates and tend to become unevenly distributed. Also, a problem arises in that spacer particles having diameters larger than the center particle diameter are easily broken. Specifically, if polymer particles obtained by the polymerization of only a monofunctional vinyl monomer such as styrene are used as a spacer, these polymer particles are easily broken or distorted by pressure when a clearance between two substrates is set because these particles generally have low mechanical strength.
In order to solve this problem, introducing a crosslinked structure into polymer particles has been proposed. If a polymer particle has a crosslinked structure, distortion of the particle due to heat is prevented, and since the polymer is also prevented from being dissolved when used in a solvent, the polymer particle can be widely used irrespective of whether the solvent is water or an organic solvent.
With regard to the production of polymer particles having a crosslinked structure, various methods are known. As a typical method, a method of introducing a crosslinked structure into a polymer particle by adding a crosslinking monomer, and specifically a polyfunctional monomer, to a polymerizable monomer has been proposed. Examples of a granulating method include emulsion polymerization and suspension polymerization. While emulsion polymerization makes it possible to obtain polymer particles having a good distribution of particle diameters, it is generally difficult to produce large particles having an average particle diameter of 1 μm or more.
Therefore, studies are being carried out to improve the distribution of the particle diameters of polymer particles. Japanese Patent Application Laid-Open (JP-A) Nos. 58-106554 and 63-191818 propose a so-called seed polymerization method in which polymer particles are first prepared by emulsion polymerization and then the polymer particles are used as a seed to carry on polymerization, thereby promoting growth of the particles to enlarge the particles. However, this seed polymerization method involves a production process with two or more steps and there has the drawbacks of inferior operability and economy.
As a method of producing polymer particles by one step, a method of producing monodispersion vinyl polymer particles by polymerizing one or more vinyl monomers in an organic solvent or a water/organic solvent in the presence of a specific dispersion stabilizer is disclosed, for example, in JP-A No. 63-191805. Further a method of producing a fine particle-like crosslinked type N-vinylcarboxylic acid amide resin having an average particle diameter of 10 μm or less by polymerizing one or more vinyl monomers in a non-aqueous solvent in the presence of a crosslinking agent is disclosed in JP-A Nos. 4-323213 and 10-310603.
However, the polymer particles obtained using the method described in JP-A No. 63-191805 have a wide particle size distribution and are therefore not necessarily satisfactory. Also, although an average particle diameter of the resulting resin fine particles is indicated in JP-A No. 4-323213, there is no disclosure as to a distribution of particle diameters. JP-A No. 10-310603 discloses polymer particles having such a narrow particle size distribution that a coefficient of variation is 2.28%. These particles, however, have an average particle diameter as small as 3 μm.
According to the findings of the inventors of the invention, in the case of using a polyfunctional vinyl monomer as a crosslinking monomer in a dispersion polymerization method performed in a non-aqueous solvent in the presence of a dispersion stabilizer, flocculation between particles occurs, and therefore, stable polymer dispersion cannot be obtained in many cases. It is therefore very difficult to obtain particles having an average particle diameter of 5 μm or more and a narrow distribution of particle diameters.